Add DenseVectorField

[BoPeng]

Per discussion at #356 , django-elasticsearch-dsl lacks a DenseVectorField corresponding to the DenseVector field defined in elasticsearch-dsl

https://github.com/elastic/elasticsearch-dsl-py/blob/579f57205c395e17024d9ae827cbf6fd626969c4/elasticsearch_dsl/field.py#L392-L397

This PR adds this field type.

The patch has been used as

@registry.register_document
class MyClassDocument(Document):

    embedding = fields.DenseVectorField(attr="get_embedding")

where get_embedding is a function that returns a list of embeddings. Embeddings returned in array type does not work because

django-elasticsearch-dsl/django_elasticsearch_dsl/fields.py

Lines 89 to 90 in e453aff

	if instance == field_value_to_ignore:
	return None

will fail if an array is compared to None.